Here is how to share a VM between Windows-based VMWare Workstation and Mac-based Fusion:

Create a large FAT32 partition. You can either carve up your primary hard drive using something like Partition Magic – or do something more sane like buy external Firewire drive [USB drive performance on OS X is abysmal]. I own this Firelite drive which is powered over Firewire and also this Firewire 800 G-Tech drive. Let me re-emphasize: get a Firewire drive, USB is painfully slow.

Format the drive using Disk Utility with the ‘MS-DOS’ filesystem. Windows, for no apparent reason, refuses to format a FAT32 volume larger than 32GB – so the format must be done in Disk Utility.

FAT32 is limited to 4 GB files, so you’ll need to make sure your virtual disk is split in to into 2GB segments. It’s easy to specify this option during VM creation or you can convert an existing VM with the command line VMWare disk utilities. I recommend Robert Petruska’s DiskManager GUI, which makes things much easier. I recommend copying the virtual disk to a local drive first, it’ll save a lot of time.

Modify the VM configuration to point at the split disk you just converted, and you’re good to go!

The only real drawback is that Fusion cannot do much [anything] with the tree of snapshots in created in workstation.